The pumping action of the heart is controlled in an orderly manner by electrical stimulation of myocardial tissue. Stimulation of this tissue in the various regions of the heart is controlled by a series of conduction pathways contained within the myocardial tissue. The impulse to stimulate is started at the sino-atrial (SA) node and is transmitted through the atria. The signals arrive at the atrio-ventricular (AV) node which is at the junction of the atria and ventricles. The signal passes through the AV node into the bundle of HIS, through the Purkinje fiber system and finally activates the ventricular muscle. At the completion of ventricular stimulation, heart tissue rests to allow the cells to recover for the next stimulation. The stimulation is at the cellular level, and is a changing of the polarity of the cells from positive to negative.
Cardiac arrhythmias arise when the pattern of the heartbeat is changed by abnormal impulse initiation or conduction in the myocardial tissue. The term tachycardia is used to describe an excessively rapid heartbeat resulting from repetitive stimulation of the heart muscle. Such disturbances often arise from additional conduction pathways which are present within the heart either from a congenital developmental abnormality or an acquired abnormality which changes the structure of the cardiac tissue, such as a myocardial infarction.
One of the ways to treat such disturbances is to identify the conductive pathways and to sever part of this pathway by destroying these cells which make up a portion of the pathway. Traditionally, this has been done by either cutting the pathway surgically, freezing the tissue, thus destroying the cellular membranes, or by heating the cells, thus denaturing the cellular proteins. The resulting destruction of the cells eliminates their electrical conductivity, thus destroying, or ablating, a certain portion of the pathway. By eliminating a portion of the pathway, the pathway may no longer maintain the ability to conduct, and the tachycardia ceases.
One of the most common ways to destroy tissue by heating has been the use of electromagnetic energy. Typically, sources such as radiofrequency (RF), microwave, ultrasound, and laser energy have been used. With radiofrequency energy, a catheter with a conductive inner core and a metallic tip are placed in contact with the myocardium and a circuit is completed with a patch placed on the patient's body behind the heart. The catheter is coupled to a radiofrequency generator such that application of electrical energy creates localized heating in the tissue adjacent to the distal (emitting) electrode. Because of the nature of radiofrequency energy, both the metallic tip and the tissue are heated simultaneously. The peak tissue temperatures during catheter delivered application of RF energy to myocardium occur close to the endocardial surface, such that the lesion size produced is approximately limited by the thermodynamics of radial heat spread from the tip. The amount of heating which occurs is dependent on the area of contact between the electrode and the tissue and the impedance between the electrode and the tissue. The higher the impedance, the lower the amount of energy transferred into the tissue.
One of the major problems with radiofrequency energy is the coagulation of blood onto the tip of the catheter, creating a higher impedance or resistance to passage of electrical energy into the tissue. As the impedance increases, more energy is passed through the portion of the tip without coagulation, creating even higher local temperatures and further increasing coagulum formation and the impedance. Finally enough blood is coagulated onto the tip such that no energy passes into the tissue. The catheter must now be removed from the vascular system, the tip area cleaned and the catheter repositioned within the heart at the desired location. Not only can this process be time consuming, but it may be difficult to return to the previous location because of the reduced electrical activity in the regions which have been previously ablated. Use of temperature sensors in the tip to modulate the power input to keep the electrode below the coagulation temperature of blood have been used. These systems inherently limit the amount of power which can be applied. Others have used closed loop cooling systems to introduce water into the tip, but these systems are larger than necessary because the coolant must be removed from the catheter.
Increase of impedance was noted in radiofrequency (RF) ablation at power levels above 7 watts (W) due to the formation of a thin insulating layer of blood degradation products on the electrode surface. Wittkampf, F. H. et al., Radiofrequency Ablation with a Cooled Porous Electrode Catheter, Abstract, JACC, Vol. 11, No. 2, Page 17A (1988). Wittkampf utilized an open lumen system at the distal electrode which had several holes perpendicular to the central lumen which could be cooled by saline. Use of the saline kept the temperature of the electrode at a temperature low enough so that the blood products would not coagulate onto the tip of the electrode.
Impedance rise associated with coagulum formation during RF catheter ablation was also noticed by Huang et al., Increase in the Lesion Size and Decrease in the Impedance Rise With a Saline Infusion Electrode Catheter for Radiofrequency Catheter Ablation, Abstract, Circulation, Vol. 80, No. 4, page II-324 (1989). A quadropolar saline infusion intraluminal electrode catheter was used to deliver RF energy at different levels.